The invention relates generally to condition assessment and more particularly to condition assessment of electrical machines.
Many different types of electrical machines use electrical conductors that are insulated. For example, motors and generators commonly include wound rotors and stators or may include preformed windings or coils. The conductors typically serve either to conduct electrical current or to produce magnetic fields by virtue of the flow of such current. Insulation systems separate conductors and windings from one another and from adjacent components in the assembled system. Such insulation systems may include various varnish systems, tapes, coatings, and sleeves. The integrity of the insulation systems is important to the reliability and life expectancy of the electrical equipment in which they are installed.
Off line methods for evaluating the insulation condition include over-voltage, high potential, or high voltage ramp tests, insulation resistance or polarization index tests, surge tests, dissipation factor tests, also known as “tan delta” or power factor tests, and off-line partial discharge tests. Conventional off-line techniques can be intrusive and costly because the electrical machines must be shut down and taken out of service to perform the required diagnostic test.
Several on-line measurement techniques are also available for monitoring the winding insulation condition. Techniques such as vibration measurement and temperature measurement detect severe fault conditions only after the fault has occurred and result in proactive maintenance and servicing being difficult. Partial discharge analysis techniques detect early symptoms of insulation degradation but require expensive specialized equipment. Typically on-line measurement techniques are combined such that a different sensing instruments and packages are used for assessing different parameters. In particular, resistive temperature sensors typically comprise conductors around an insulated base which are wound and then molded. The resulting package is bulky and not easily integrated with other sensor packaging.